A variety of mental health factors have been implicated in the etiology of physical disease. Many investigators have searched for personality and behavioral variables that would help to explain these disease processes. One of the few stress-behavior or mental health characteristics that has definitely been established as carrying important health risk, in normal and near-normal individuals, is what is known as Type A behavior pattern, which predisposes to coronary heart disease. This concept has been formally developed and validated in recent years by Rosenman and Freidman, but its origins go back at least as far as Osler (in 1892). In the last few years, as the relationships of psychological stress and health have been given increasing emphasis, the Type A behavior pattern has also received increasing emphasis. A growing number of proposals for studies of interventions on this behavior pattern are being submitted, and some of these proposed efforts are quite sizeable. This tends to create problems, since the Type A behavior pattern is well established as a risk factor but is not really very well defined as an entity. It is presently scored on the basis of a global, subjective assessment of impressions from a short interview. Several Type A questionnaires exist, but all have discriminating power that is substantially inferior to that of the interview. The interview technique has inherent limitations in regard to its objectivity, replicability and functional effectiveness. Because of its global nature, the interview simply cannot be very illuminating of Type A behavior in all of its real complexity. There is reason to believe that Type A behavior is a multi-dimensional phenomenon. It is unclear how the various facets of Type A behavior enter into the definition of risk, and how they should be addressed in an intervention program. An epidemiological analysis of the Type A stress-behavior pattern that addresses these concerns is proposed.